Some communication systems, commonly referred to as Multiple-Input Multiple-Output (MIMO) systems, transmit and receive simultaneously using multiple transmit and receive antennas. Some MIMO techniques transmit phase-shifted replicas of a transmitted signal from multiple antennas to produce a directional, high-gain transmission. These techniques are often referred to as Beam Forming (BF) techniques. The phase differences between the signals transmitted from the different antennas determine the direction of the transmission. Thus, beamforming systems are often sensitive to undesired phase and frequency offsets between signals.
Several methods are known in the art for measuring and/or correcting phase and frequency offsets in MIMO systems. For example, U.S. Patent Application Publication 2006/0135077, whose disclosure is incorporated herein by reference, describes a multi-transceiver system adapted to estimate a frequency offset on the basis of a test signal and a reference signal. Transceivers in the multi-transceiver system are adapted to transmit signals compensated with the frequency offsets.
U.S. Patent Application Publication 2006/0209979, whose disclosure is incorporated herein by reference, describes a method of tracking receiver frequency offsets in a receiver of a MIMO system. The frequency offsets due to each of a number of receiver sub-systems are estimated by monitoring frequency offsets on a number of channels or sub-carriers on different frequencies. The channel frequency offsets are preferably estimated by detecting the phase rotation between adjacent pilot symbols on each respective channel.